I have a spare unused Oxted plastic seat base tray which was for a 2002 car. The seat base trays are the same for S3 and SV.
I was going to use it to make myself a new 2-part PU foam seat (taking into account canoeing lessons learned obviously) but the bit of the tray that slots into the seat frame is about 1cm wider on this spare tray than the ones currently used in my 2016 vintage SV. So this available seat tray is too wide for my new seat frame; where it slots into the seat frame the tray is 24cm wide at the front and 27.5cm at the back (for comparison, my 2016 car's seat tray is 23cm and 26.5cm)
So I've ordered one that fits my 2016 seat frame from Oxted and have this spare one for sale - £10 plus postage at cost if required. NB these are £20 with L7C discount from Oxted